1. Field of the Invention
The present invention relates to a printing apparatus and a residual ink detection method and, more particularly, to, for example, an inkjet printing apparatus having an arrangement for detecting residual ink in an ink tank using an electrode and a residual ink detection method.
2. Description of the Related Art
In recent years, inkjet printing apparatuses (to be referred to as printing apparatuses hereinafter) have been widespread, each of which discharges ink from the nozzles of a printhead to a printing medium such as a printing paper sheet and applies the ink onto the printing medium to form dots, thereby performing printing. Each nozzle of the printhead is provided with a discharge energy generation element such as a heater or a piezoelectric element. The nozzle is made to discharge ink by applying an electrical signal to the element.
In such a printing apparatus, the ink to be consumed by the printhead is supplied from an ink tank attached to the printing apparatus main body. Hence, when the printing apparatus continues printing regardless of a shortage of the ink amount in the ink tank, the ink cannot be discharged in a regular discharge amount, and a fault such as density unevenness occurs in the printed image. Especially in a thermal printing apparatus that discharges ink using a heater, if an electrical signal is applied to the heater in a state in which no ink remains in the printhead, the load of discharge energy may directly be applied to the nozzle surface, and the head may malfunction.
To solve this problem, such a printing apparatus conventionally has an arrangement for detecting a low residual ink level in the ink tank. Detecting a low residual ink level in the ink tank will be referred to as residual ink detection hereinafter. As the residual ink detection method, an optical method, a mechanical method, and an electrode method have been put into practical use. In particular, the electrode method is widely used because it need not a complex arrangement and can be manufactured at low cost.
Residual ink detection by the electrode method measures a voltage value generated upon supplying an electric current to a pair of electrodes inserted into the ink tank, and compares the detected voltage value with a threshold voltage provided for the detected voltage, thereby detecting the presence/absence of ink. In the residual ink detection by the electrode method, however, if bubbles generated when supplying ink to the printhead remain in the ink tank, the electric current may go through via the bubbles between the electrodes even if no ink exists in fact. Hence, the detected voltage may exhibit a value similar to that in the presence of ink, and the presence of ink may erroneously be detected although no ink remains in fact.
For example, an air communicating ink supply system introduces the air, thereby supplying ink to the printhead while maintaining pressure equilibrium. The outside air introduced at this time along with ink consumption normally changes to bubbles in the ink layer of the ink tank, and the bubbles dissipate upon moving to the air layer. However, for example, when high-duty images are continuously printed, the bubbles may remain in the ink tank without dissipating upon moving to the air layer. Such bubble formation in the ink tank may occur even in a supply system of another type.
Japanese Patent Laid-Open No. 2007-268805 proposes a method of measuring a voltage value at time 1 and time 2 (time 1<time 2) during an energization period, providing threshold voltages to the respective times, and comparing the voltage value with threshold voltages, thereby discriminating three states: an ink present state, an ink absent state, and a bubble present state.
Japanese Patent Laid-Open No. 2007-268805 utilizes a characteristic representing that a stepwise voltage characteristic is obtained with respect to the energization time in the bubble present state. In the bubble present state, no large difference is generated with respect to the voltage characteristic in the ink present state during the normal measurement time. For this reason, if the voltage value exhibits an intermediate value within a certain range at time 1, energization is continued up to a time at which the voltage value abruptly increases. The voltage is measured at time 2, and it is confirmed whether or not the voltage value exceeds a predetermined threshold voltage. If the voltage value exceeds the threshold voltage, the bubble present state is determined, and the ink is regarded to be absent. In addition, even when the ink is consequently regarded to be present, if the voltage value at time 1 exhibits the intermediate value a plurality of times upon repetitively executing residual ink detection, the ink is regarded to be absent regardless of the voltage value at time 2.
However, during the energization period, the voltage value gradually increases along with the elapse of time in any of the ink present state and the bubble present state, and the voltage is saturated. It is therefore difficult to observe a non-conducting state between the electrodes by prolonging the energization period. Additionally, from the viewpoint of the measurement period, prolonging the energization period means requiring a long time until completion of measurement of inks of all colors. Furthermore, in the method of repetitively detecting the intermediate potential to discriminate the bubble state, re-execution of residual ink detection is necessary. Hence, time is required until a discrimination result is obtained.